Narrow pulse generation

I'm pretty dense here, so it's not yet becoming obvious to me how something like this works. I can't begin to fathom how a quad analog switch will let you generate thousands of precisely different time delays (different R values) to trigger the ADC.

I'll be doing this with my old analog scope, probably only at night. ;-)

Usually it's just a linear analog ramp feeding a comparator, with the other side of the comparator driven from a dac to set the delay. With reasonable layout and such, jitter of 1 part in 50,000 of max delay isn't hard.

High-end sampling scopes use a triggered oscillator to tick off coarse delays, with a ramp/comparator for fine delays. TDR really doesn't need that.

John

Ding, the light goes on. :-) I see, so you just trigger the ramp when you "step" the output signal. Pretty slick. Ok, I think I can envision how this works and how to drive it with a cheap micro. I know the reflected signal can come back with the opposite polarity, so how do you get the ADC to cope with that. Just float the circuit up from ground with a couple of diodes so that the ground ref for the ADC is below ground level for the signal? Sorry for making you design an entire product, but it has always intrigued me how they can build a hand-held TDR with a numeric digital display for so cheap. I think I see now. :-)

Eventually, anything that's worth doing gets integrated.

5 volts into 50 ohms in 640 ps, 26 cents.

John

It's used as controlled resistors. But that chip has become nearly unobtanium so you'd be pretty much back to single FETs. If you happen to have a fast small FET you can try it out. You'll need something like a BF998 or another from that series. In a pinch they can be found in TV tuners. Who needs TV anyhow...

For simplicity use a L-R where the FET is the R. 330nH or something around that value connected to the output of the inverter where the fast transition comes from. Drain to other side of the cap, source to GND, then place a variable DC voltage onto both gates (or to the one if it's a single-gate FET). Input of another inverter to the drain. Now connect probes to both digital outputs and see one of them scoot when you vary the gate voltage. I don't know whether your Hitachi scope is able to do that though, I use a 1GHz scope for that kind of stuff.

Regular switching FETs such as the 2N7000/7002 aren't very suitable. Too much capacitance.

If it's one of those 100MHz Hitachis don't expect much in time resolution. Other than that they are good scopes.

--
Regards, Joerg

http://www.analogconsultants.com

Sweet! Sometimes a digital chip comes along that can make a mostly analog job a lot easier.

--
Regards, Joerg

http://www.analogconsultants.com

Very nice design (and nice web pages too). The avalanche is the best methode for generating fast rising edge pulses, but the problem is syncronisation with an external trigger which is almost impossible .

The load variation is changing dramatically the slew rate, so a request like load from open circuit to dead short, 5nS pulses (which will be the rise time ?) and 10V amplitude...

best wishes, Vasile

Sorry, I'm sure the other readers (except myself) knows what FWHM means:

The TDR needs a very short rise time and as high amplitude as possible. The pulse lenght could be higher than 5nS.

A toy doing some "TDR" is here:

Vasile

Avalanche transistors are easily triggered, with picosecond jitter. Generations of sampling oscilloscopes, from Lumatron up through the Tek 7000 series, used triggered avalanche transistors to drive their sampling gates, directly or through an srd. Nowadays, they mostly use SRDs or shock lines driven by non-avalanche stuff.

HP tended to not use avalanche transistors in their samplers, at least after the srd was discovered (with, ironically, an avalanche-based sampler!)

John

Interesting. I just learned something. Found this link to an early HP paper on the general subject:

Thanks, Jon

The story (I have it in an old HP Journal somewhere) is that some guy at HP was experimenting with a diode-based frequency multiplier, and found he was getting far stronger harmonic output than theory predicted. They had just introduced the HP185 sampling scope (700 MHz bw, using an avalanche transistor in the sampling gate) so dragged it over to see what was happening, thus discovering the SRD. Shortly after that, the 185 used an srd instead of the avalanche transistor, increasing its bandwidth to 1 GHz. A little later they introduced the

187 plugin, with almost 4 GHz bandwidth, still largely tube-based. This all started about 1962.

I have the Journal article, and the 185 manuals, and an HP185 scope itself; historical and very, very ugly.

John